All-trans-zeta-carotene desaturase (EC 1.3.99.26, Crtlb, phytoene desaturase (ambiguous), 2-step phytoene desaturase (ambiguous), two-step phytoene desaturase (ambiguous), CrtI (ambiguous)) is an enzyme with systematic name all-trans-zeta-carotene:acceptor oxidoreductase.^[1] This enzyme catalyses the following chemical reaction

all-trans-zeta-carotene + 2 acceptor ${\displaystyle \rightleftharpoons }$ all-trans-lycopene + 2 reduced acceptor (overall reaction)

(1a) all-trans-zeta-carotene + acceptor ${\displaystyle \rightleftharpoons }$ all-trans-neurosporene + reduced acceptor

(1b) all-trans-neurosporene + acceptor ${\displaystyle \rightleftharpoons }$ all-trans-lycopene + reduced acceptor

This enzyme is involved in carotenoid biosynthesis.